Relative Comparison of Additive Manufacturing: Inconel 718

Arnob Banik

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Additive Manufacturing

• Known as 3D printing
• A group of manufacturing processes that build components in an additive layer-by-layer or drop by-drop fashion, which is able to obtain net shape components.
• Freedom to design and manufacture geometrically complex structures that are either impossible or considerably expensive by conventional processes, which rely on removing materials from monoliths

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Inconel 718 (IN718)

• A nickel-base superalloy
• Developed by International Nickel Company in 1959
• High strength, good weldability and fabricability
• Good mechanical properties up to 650 ◦C and competitive price due to its low cobalt and high iron content
• High temperature applications
• Used in Aerospace, gas turbine, nuclear, oil & gas industry

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Alloying elements

• Principal Element – Ni
• Forms the austenitic fcc matrix phase γ
• Addition of Al, Ti and Nb would tend to form strengthening precipitates γ′ and�γ′′ ��

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Phases in IN718

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• γ′ and γ′′
• γ′ and γ′′ are present and coherent with the γ matrix
• Both γ′ and γ′′ can strengthen the γ matrix
• δ
• γ′′ phase in IN718 is metastable and can covert to the thermodynamically stable δ phase
• δ phase is incoherent with γ matrix, conferring rarely strength to the matrix when present in large quantities
• Precipitation of δ phase would be at the expense of Nb, which associates with the loss of γ′′ and therefore strength
• MC carbide
• Predominant carbide phase in IN718 is Nb-rich MC phase
• Intragranular MC carbide can impede basic dislocation movement but confer very small strength to the matrix
• MC carbides precipitated at grain boundaries promote the fracture mode transiting from transgranular to intergranular at room temperature
• Laves
• Laves phase is a brittle intermetallic topologically close packed (TCP) phase, and�is detrimental to mechanical properties �� �������

Manufacturing IN718

• Wrought IN718
• Wrought is a process that mechanically works a cast billet or ingot several times at high temperature to get the final product.
• Wrought microstructure is generally more homogeneous and has finer grains than cast microstructure.
• Cast IN718
• Castings are intrinsically stronger than forgings at elevated temperature since the coarser grains in castings favor high temperature strength
• Microstructure considerably different from that of wrought form
• Powder metallurgy (P/M) IN718
• Powder metallurgy (P/M) approach is able to produce finer grains, more uniform properties and near-net-shape components.
• Had been proposed to produce integral IN718 turbine rotors for space vehicles with short life but subjected to high temperature and high stresses
• Additively manufactured IN718
• Electron beam melting (EBM)
• Selective laser melting (SLM)
• Binder jet 3D printing (BJ3DP)

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Electron Beam Melting

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https://www.tandfonline.com/doi/full/10.1080/09506608.2016.1176289

• EBM is a powder-bed fusion (PBF) process that uses electron beam to selectively�melt the defined geometries at each layer and simultaneously fuses with previously�solidified layers in a powder bed, by which method a 3D part is built
• Scanning of electron beam is ifrstly to preheat and slightly sinter the current powder layer, then melt the current powder layer and form a solid layer of the build.

Selective Laser Melting (SLM)

• After the component’s 3D model has been processed and transferred to the laser-base AM machine, the building process starts with applying a thin layer of powder on the building base plate. A laser beam is then used to melt the powders at the locations defined by design data for the current layer, followed by lowering the base plate and applying a new layer of powder with the identical thickness on the top

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www.eos.info/additive_manufacturing/for_technology_interested

Microstructure�EBM

• Building Direction
• Heterogeneous morphology
• Contour region – �Parallel to building direction - equiaxed (outermost) ; slim (centre) and wide(overlapping)width columnar grains�Perpendicular -
• Hatch Region – Columnar grain (parallel) ; equiaxed (perpendicular to BD)

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SEM micrographs showing the microstructures in the contour region imaged (a) parallel to and (c) perpendicular to building direction (BD), and in the hatch region imaged (b) parallel to and (d) perpendicular to building direction (BD)

Microstructure�SLM

• Along BD-
• Irregular shape, grains tilted away from the building direction in random angles
• Perpendicular to BD
• Randomly oriented

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SEM micrographs showing the unetched microstructures of the as-manufactured vertically built (a) and horizontally built (b) samples, (c) and (d) are the magnified areas marked in (a) and (b) respectively

Schematic depicting the geometry of as-manufactured�block

δ phase - EBM

• investigate the precipitating behaviors of δ phase at the grain boundaries, both high angle grain boundaries (>15°) and low angle grain boundaries (2–15°) are determined by ESBD mapping
• Very few and small needle-like δ phase (less than 100 nm) can only be found at the high angle grain boundaries,
• while low angle grain�boundaries are free of δ phase

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Precipitates in as-manufactured sample characterized parallel to the building direction: (a) BSE image showing the generally distribution of precipitates in the sample, (b) magnified area indicated in (a), (c) EDS spectrum of precipitate indicated in (b), (d) IPF colouring map identifying low angle (white colour, 2–15°) and high angle (black colour, >15°) grain boundaries of this interested area, (e) grain boundary morphology corresponding to the low angle grain boundary indicated in (d), and (f) grain boundary morphology corresponding to the high angle grain boundary indicated in (d).

γ″- EBM

• Existence of very fine and coherent precipitates embedded in the γ matrix (a)
• (b) Elongated reflection spots are derived from the elongation of γ″ in its c crystallographic axis, and are stronger than the dot-like reflections of γ′
• Morphology of γ′ is hemispheroidal and has dot-like reflection
• 5(c) & (d) very few small hemispherical γ′ can be found in the γ″(110)/γ′(011) dark-field image while disappear in γ″(011) dark-field image

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TEM images showing as-manufactured microstructure in the hatch region: (a) Bright-field image showing coherent precipitates in the γ matrix; �(b) Diffraction pattern of [001] zone of γ matrix exhibiting γ′ and γ′' superlattice reflections; (c) Dark-field image using the γ″(110)/γ′(011) diffraction spot; (d) Dark-field image using the γ″(011) diffraction spot; (e)

HRTEM image and the corresponding FFT diffractogram of the TiN precipitate

Precipitates – SLM

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(a)TEM bright field micrograph of the as-manufactured horizontally built sample and the inserted is the diffraction pattern from the corresponding area. (b)high-resolution TEM taken from the dash-square area indicated in (a) and the FFT (Fourier transformation). (c)TEM bright field micrograph of the as-manufactured vertically built sample and the inserted is the diffraction pattern from the corresponding area.

• High density of dislocations can be seen in both horizontally built and vertically built samples- attributed to the large plastic strain inherited from the manufacturing process
• a and c is indexed as γ matrix taken from the [001] zone axis.
• diffraction pattern does not show any diffraction spots from γ ‘/γ ″ , indicating the absence of principal strengthening phases in the as-manufactured condition.
• From the FFT diffractogram -Laves phase from [2423]

Heat Treatment

• FC, AC and WC denotes furnace cooling, air cooling and water cooling, respectively. H2 is the standard heat treatment for wrought IN718
• AS, DA, SA, HA and HSA are the abbreviations of as-manufactured, direct ageing, solution + ageing, homogenization + ageing and homogenization + solution + ageing, respectively
• FC, AC and WC denotes furnace cooling, air cooling and water cooling, respectively. SA is the standard heat treatment for wrought IN718 (AMS 5662). HSA is the standard heat treatment for cast IN718

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Hardness Test

• A Struers DuraScan G5 hardness tester
• Vickers microhardness with 300 g load and 15 s dwell-time.
• For each sample no less than 20 indentations were performed to get good statistics.

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• EBM- hardness in as-manufactured condition is relatively high (HV0.3 427.5), comparing with the minimum hardness requirement (HV 350) of AMS 5662. Heat treatments can increase the hardness, among which direct ageing without solution yields the highest hardness
• SLM- Due to the absence of strengthening phases γ ‘/γ ″ , the as-manufactured samples give a relatively low hardness (HV0.3 325), which does not even satisfy the minimum hardness (HV 350) requirement per AMS 5662 standard
• Heat treatments can considerably increase the hardness compared to the as-manufactured condition, the hardness is nearly the same among these four heat-treated conditions

Tensile Test

• To investigate the sample orientation dependency of tensile properties
• 3∼4 samples were tested per test condition to obtain acceptable statistics.
• Tensile tests were conducted under room temperature and open air
• Instron 5582 universal test machine with a 100 kN load cell and at a 0.10 %/s strain rate

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EBM

• Ultimate tensile strength and yield strength tested
• parallel to the building direction are in general higher than that tested perpendicular to building direction
• Elongation and Young’s and modulus show the opposite tendency
• Heat treatments improve the yield strength and ultimate tensile strength- attributed to precipitate strengthening phases of γ′ and γ″ during ageing
• All ultimate tensile strengths for as-manufactured and heat-treated conditions tested in both directions fail to meet the AMS 5662 minimum requirement (1275 MPa)
• Elongations qualify for the corresponding minimum requirement

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SLM

• Heat treatments can significantly
• increase the yield strength and ultimate tensile strength
• decrease the elongation at failure.
• Low tensile strength - attributed to the absence of strengthening phases γ′and γ″
• Yield strength and ultimate tensile strength are higher in the horizontally built sample
• The sample orientation dependence of mechanical properties decreases after heat treatments – from partial or complete removal of residual stress and the dislocation recovery

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Residual Stress- EBM

• Residual stress maps of the unsectioned EBM sample. (a-c)
• Residual stress maps for σx, σy, and σz respectively using the average d-spacing of all six reference blocks for each sample orientation/strain direction. (d-f) Residual stress maps for σx, σy, and σz respectively using the average d0 values generated after force and moment balance. �

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Residual Stress - SLM

• SLM method does not include a preheated base plate or a preheating step before the melt phase in the processing
• Relative trend of residual stresses shows compressive stress near the center of the build in an ellipsoid distribution

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Comparison

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WHICH IS BETTER?

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Powder Bed Binder Jet 3D Printing

• Binder Jetting technology for 3D printing complex parts in industrial-grade materials.
• A liquid binding agent is selectively deposited to join powder particles.
• Layers of material are then bonded to form an object.
• The printhead strategically drops binder into the powder.
• The job box lowers and another layer of powder is then spread and binder is added.
• Over time, the part develops through the layering of powder and binder. 
• Binder Jetting is capable of printing a variety of materials including metals, sands and ceramics
• Other materials are typically cured and sintered and sometimes infiltrated with another material, depending on the application.

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Schematic of binder jet 3D printing process.

Conclusion

• BEM – Better without heat treatment
• SEM – Perform better after heat treatment
• Binder Jet – A good potential to have more precise production

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Future Work

• Evaluation of cyclic mechanical properties – application involve cyclic loading
• Investigation of mechanical properties from binder jet 3D printed IN718

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References

• D. Deng, J. Moverare, R. L. Peng, H. Söderberg (2017). Microstructure and anisotropic mechanical properties of EBM manufactured Inconel 718 and effects of post heat treatments. Materials Science and Engineering: A, 693,151-163
• D. Deng, R. L. Peng, H. Brodin, J. Moverare (2018). Microstructure and mechanical properties of Inconel 718 produced by selective laser melting: Sample orientation dependence and effects of post heat treatments. Materials Science and Engineering: A, 713, 294-306
• Peeyush Nandwana, Amy M. Elliott, Derek Siddel, Abbey Merriman, William H. Peter, Sudarsanam S. Babu (2017). Powder bed binder jet 3D printing of Inconel 718: Densification, microstructural evolution and challenges, Current Opinion in Solid State and Materials Science, 21(4),207-218
• Sochalski-Kolbus, L.M., Payzant, E.A., Cornwell, P.A. et al.(2015). Comparison of Residual Stresses in Inconel 718 Simple Parts Made by Electron Beam Melting and Direct Laser Metal Sintering Metallurgical and Materials Transactions A ,46(3), 1419-1432
• Arcam EBM. EBM Hardware. http : / / www . arcam . com / technology /electron-beam-melting/hardware/
• EOS. Additive Manufacturing, Laser-Sintering and industrial 3D printing - Benefts and Functional Principle. https://www.eos.info/additive_manufacturing/for_technology_interested.

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Thank You